The invention relates to a hydraulic power steering system.
Passenger cars and commercial vehicles of today usually are equipped with hydraulic or electrohydraulic power steering. The steering process of a driver is supported by the force produced by an actuator. The hydraulic energy is produced by a pump, which is driven permanently by the internal combustion engine of a motor vehicle. The pump output must be designed so that large steering angle changes can be carried out in a short time at low revolutions of the engine when the vehicle is stationary. On the other hand, when driving at higher speeds, a comparatively low steering energy is required, so that the pump and the steering system are overdimensioned in this case. This leads to a considerable dissipation of power in the steering system, which is reflected in an increased temperature of the pressure medium.
Supporting the moment or boosting the steering as a function of the speed is known.
DE 38 08 900 A1 describes a steering system for a motor vehicle with a hydraulic pump, which is driven by an electric motor and acts upon a steering cylinder by way of a steering valve. The electric motor and the hydraulic pump are activated only upon input of a nominal steering angle at a steering handle of the motor vehicle and otherwise are switched off in order to save energy.
DE 43 07 864 A1 describes a power steering system for motor vehicles with a hydraulic pump, which is driven by an electric motor and which can be connected over hydraulic lines by means of a control valve with at least one working chamber of a hydraulic cylinder, which is coupled to a mechanical steering system to support the steering power. When no steering support is required, the electric motor is operated with reduced power in a stand-by mode. When steering support is required, the electric motor is operated in an operating mode.
DE 102 56 306 A1 discloses a hydraulic power steering system for a vehicle with a steering gear and with a first hydraulic actuator for supporting the operation of a steering handle by a driver in order to bring about a shift in the steering angle of steered wheels of a vehicle. In order to bring about a functional gain in the hydraulic power steering with the aim of varying the amplification performance curve of the power steering, a second, preferably electric actuator is added, which is constructed as a steering shaft actuator.
The known electrical power steering systems are not suitable for commercial vehicles, since the electric power required is not adequate. When a second actuator is added to a hydraulic power steering system, the power required by the hydraulic steering system basically is not reduced, since the first actuator and its supplying source of pressure medium remain in operation constantly, so that the known power steering systems either are unsuitable for commercial vehicles or do not have any structural distinguishing features for clearly reducing the energy required during their operation.